[{"_id":"1","prop1":"...","prop2":"...","someArray":[{"value":"sub element 1.1"},{"value":"sub element 1.2"},{"value":"sub element 1.3"}]},{"_id":"2","prop1":"...","prop2":"...","someArray":[{"value":"sub element 2.1"},{"value":"sub element 2.2"}]}]

db.collection.aggregate([{"$addFields":{"someArray":{$function:{body:"function(arr) {return arr.map(function(elem){elem['_id'] = new ObjectId(); return elem;})}",args:["$someArray"],lang:"js"}}}}])